As radio waves have been utilized in wider applications, it has been a challenge to balance avoiding interference and efficient frequency usage. For early detection of radio wave interference and sharing of radio wave resources, a technique has been proposed in which sensing of radio waves is performed to establish a radio wave environment database (NPL 1). The object of the technique is to detect interference as well as an available frequency band, and to facilitate utilization of frequency resources in secondary application by analyzing the radio wave environment database.
For early detection of interference or change in status of radio wave usage, it is necessary to densely dispose radio wave sensors and to frequently monitor the radio wave environment with respective radio wave sensors. Radio wave information to be monitored typically includes a received signal strength indicator (RSSI).
PTL 1 also discloses a technique for obtaining strength of a radio wave at a plurality of discrete observation points by calculation or measurement to determine a distribution of radio wave intensities. Since it is difficult to detect interference of radio waves of the same frequency by using only a single parameter such as the received signal strength indicator, in the technique of PTL 1, observation is performed using a statistic parameter having a different characteristic for each signal such as a modulation method as environmental information.
PTL 2 discloses a device for detecting interference of radio waves by utilizing a fact that an amplitude probability distribution is different for each signal modulation method. A statistic parameter such as an amplitude probability distribution may be used as a parameter for identifying a feature of a radio wave, and interference or the like may be identified using the feature by comparison with a case where only the received signal strength indicator is observed.
A problem in extracting a feature of a radio wave includes signal degradation due to fading. Fading is a phenomenon in radio communications in which radio waves that have arrived with a time difference interfere with each other, which affects intensity levels of the radio waves. The time difference between signals is generated by reflection of the radio waves at an obstacle on the ground, the ionosphere, or the like. In mobile communications, movement of a terminal itself for transmitting and receiving radio waves generates fading spatially and temporally. Variation of radio wave levels due to fading causes observation of something different from a characteristic of a communication signal that is supposed to be observed with radio wave sensors, which blocks radio wave monitoring. PTLs 3 to 9 also describe techniques for monitoring radio waves.